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Electro-Magnetic Radiation and Pioneering Experiments at the SPS

Electro-Magnetic Radiation and Pioneering Experiments at the SPS. J/ . QGP. Drell-Yan. Shuryak PLB 78B (1978) 150. Shuryak 1978: Birth of the Quark Gluon Plasma. Data from 400 GeV p-A at FNAL e + e - for high mass PRL 37 (1976) 1374 m + m - for high mass PRL 38 (1977) 1331. p-A 400 GeV.

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Electro-Magnetic Radiation and Pioneering Experiments at the SPS

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  1. Electro-Magnetic Radiation and Pioneering Experiments at the SPS

  2. J/ QGP Drell-Yan Shuryak PLB 78B (1978) 150 Shuryak 1978: Birth of the Quark Gluon Plasma • Data from 400 GeV p-A at FNAL • e+e- for high mass PRL 37 (1976) 1374 • m+m- for high mass PRL 38 (1977) 1331 p-A 400 GeV Ultimately the wrong explanation, but this paper was landmark and kicked off the search for the QGP! Axel Drees

  3. Modifications due to QCD phase transition Chiral symmetry restoration continuum enhancement modification of vector mesons thermal radiation suppression (enhancement) Modern View: Lepton-Pair Physics known sources of lepton pairs Continuum sensitive to thermal radiation and chiral symmetry restoration Axel Drees

  4. Electromagnetic Radiation • Thermal “black body” or “Hawking” radiation • Real photons g • Virtual photons g* which appear as dileptonse+e-or m+m- • No strong final state interaction • Leave reaction volume undisturbed and reach detector • Emitted at all stages of the space time development • Information must be deconvoluted Thermal photons in range 1-2 GeV Axel Drees

  5. Predicted Medium Modifications of the ρ Meson momentum Axel Drees

  6. Dedicated Dilepton Experiments at CERN SPS Axel Drees

  7. Experimental Challenge Thermal radiation (and other signals of interest) compete with “cocktail” of g and l+l- from hadron decays after freeze-out • Real photons: • p0g, hg, wp0g, ... • More than 90% of photon yield • Virtual photons: • p0e+e-, h, wp0e+e- and direct decaysr,w,f e+e-, J/ye+e- ... • Semileptonic decays of heavy flavor • Drell Yan • Dileptons have mass  remove contribution from p0  more sensitive to thermal radiation than photons Precise knowledge of expected sources required Dileptons more sensitive than photons Axel Drees

  8. HELIOS-1 Dimuon Measurement in p-Be Measure contribution of known sources (cocktail) in same experiment Dimoun production consistent with known sources Earlier observations of “low mass enhancement” due to underestimated of h contribution Axel Drees

  9. HELIOS-1 Electron Pair Measurement in p-Be Same conclusion for electron pairs HELIOS-2 (same setup same people) attempted to measure dileptons in O-W and S-W but failed! Axel Drees

  10. The REAL Experimental Challenge (I) • Uncorrelated background: l+ and l- from different uncorrelated source • Veto as many of the pairs actively by finding partner • Remove remaining background statistically • Like and unlike sign combinatorial pairs. • Unlike sign background can be determined from unlike sign background, either measured (FG) or determined by event mixing (BG). • Total number of pairs related by geometrical mean. • True for e+e- since they are produced as pairs, even for different efficiencies. • For m, produced as singles, strictly true only if produced with Poisson distribution. • For different acceptance for singles or pairs need relative acceptance correction (obtained from mixed events) Axel Drees

  11. γ e- e+ e+ π0 e+ X e- π0 π0 γ e- γ The REAL Experimental Challenge (II) • Unphysical correlated background • Limited double track/hit resolution • False track match between detectors • Not reproducible by mixed events • MUST be removed from event sample • Correlated background: l+ and l- from same source but not signal • “Cross” pairs  “jet” pairs • Need case by case investigation with MC simulations and subtraction • If background produces same numbers of ++ and - - pair same method as for different pair acceptance works Axel Drees

  12. The CERES/NA45 Experiment at CERN Axel Drees

  13. Experimental setup: CERES-1 Axel Drees

  14. Target Region • segmented target • 8 Au disks • thickness: 25 mm • diameter: 600 mm • Air Cherenkov beam counters • Silicon drift chambers: • provide vertex: sz ~ 250 mm • provide event multiplicity (h = 1.0 – 3.9) • powerful tool to recognize conversions at the target Axel Drees

  15. Electron Identification: RICH RICH 2 RICH 1 • main tool for electron ID • use the number of hits per ring (and their analog sum) to recognize single and double rings Axel Drees

  16. Dielectron Analysis Strategy Axel Drees

  17. e- p r* g* e+ p Pioneering Dilepton Results form CERN CERES PRL 92 (95) 1272 with 424 citations • Discovery of low mass dilepton enhancement in 1995 • p-Be and p-Au well described by decay cocktail • Significant excess in S-Au (factor ~5 for m>200 MeV) • Onset at ~ 2 mp suggested p-p annihilation • Maximum below r meson near 400 MeV Hints towards modified r meson in dense medium Axel Drees

  18. Discovery of Low Mass Dilepton Enhancement HELIOS-3 HELIOS-3 E.Phys.J. C13 (2000) 433 Dilepton excess at low and intermediate masses well established Axel Drees

  19. Discovery of Low Mass Dilepton Enhancement NA38/NA50 E.Phys.J. C13 (2000) 69 NA38/NA50 S-U Dilepton excess at low and intermediate masses well established Axel Drees

  20. CERES-1 Final Results from Pb-Au CERES Phys.Lett.B422 (98) 405; Nucl.Phys.A661(99) 23 • Enhancement above known sources: m>200 MeV/c2: factor 2.30.19(stat)0.55(sys)0.69(decay) Axel Drees

  21. e- p r* g* e+ p Theoretical Calculation of p-p Annihilation >> 100 publications since 1995 • Low mass enhancement due to pp annihilation • Spectral shape dominated r meson • Vacuum r propagator • Vacuum values of width and mass • In medium r propagator • Brown-Rho scaling • Dropping masses as chiral symmetry is restored • Rapp-Wambach melting resonances • Collision broadening of spectral function • Only indirectly related to chiral symmetry restoration • Medium modifications driven by baryon density • Model space-time evolution of collision • Different approaches • Consistent with hadron production data • Largest contribution from hadronic phase Axel Drees

  22. Comparison of Theory and CERES Data • Vacuum r meson ( ) • Inconsistent with data • Overshoots in r meson region • Undershoots at low masses • Modification r meson • Necessary to describe data • Data do not distinguish between broadening or melting of r-meson (Rapp-Wambach) Dropping masses (Brown-Rho) • r/w to f mass region most sensitive to constrain models Clear evidence for medium modifications, data not accurate enough to distinguish models Axel Drees

  23. CERES-1  CERES-2 • addition of a TPC to CERES • improved momentum resolution • improved mass resolution • dE/dx  hadron identification and improved electron ID • inhomogeneous magnetic field  a nightmare to calibrate! Axel Drees

  24. vacuum r Brown-Rho scaling broadening of r Data from CERES-2 with TPC Upgrade PRL 91 (2003) 042301 • Data taking in 1999 and 2000 • Improved mass resolution • Improved background rejection • Results remain statistic limited • Pb-Au data from 40 AGeV Enhancement for mee> 0.2 GeV/c2 5.9±1.5(stat)±1.2(syst)±1.8(decays) • Preliminary 158 AGeV Pb-Au data • Consistent with 95/96 CERES data • Increase centrality 30%  8% Strong enhancement at lower s or larger baryon density Axel Drees

  25. CERES-2 result • the CERES-1 results persists • strong enhancement in the low-mass region • enhancement factor (0.2 <m < 1.1 GeV/c2 ) •  3.1 ± 0.3 (stat.) Axel Drees

  26. Dropping mass or broadening? CERES-2: Yield between w and f favors r broadening over dropping mass Axel Drees

  27. mee<0.2 GeV/c2 mee>0.6 GeV/c2 0.2<mee<0.6GeV/c2 CERES pT > 200 MeV/c 1995/96 2000  Nch CERES: Centrality Dependence of Enhancement • Naïve expectation: Quadratic multiplicity dependence • medium radiation  particle density squared • More realistic: smaller than quadratic increase • Volume change • Density profile (e.g. participant density) in transverse plane • Life time of reaction volume Strong centrality dependence Challenge for theory ! F=yield/cocktail Axel Drees

  28. hadron cocktail Brown-Rho scaling broadening of r 0.2<mee<0.7 GeV/c2 mee>0.7 GeV/c2 mee<0.2 GeV/c2 And what about pT dependence? • low mass e+e- enhancement at low pT • qualitatively in a agreement with pp annihilation • pT distribution has little discriminative power Axel Drees

  29. NA50Pb-Pb 158 GeV DY charm central collisions _ DY DD Intermediate Mass Region NA38/NA50 E.Phys.J. C14 (2000) 443 Large enhancement Consitent with charm enhancement by factor 3 Strong centrality dependence Axel Drees

  30. m - q q m + Comparison to Calculation of Thermal Radiation R.Rapp & E.Shuryak, Phys.Lett B473 (2000) 13 Data also consistent with thermal radiation Need direct measurement of open charm contribution Axel Drees

  31. NA60 5.6 Comparison of Data from Different Experiments • Different phase space regions • Map out new source in mT-y plane • Qualitative analysis only; data obtained from: • Different mass ranges • Different centralities • Different expected sources • Dilepton Excess • Concentrated at midrapidity • And at low pT • Consistent with p-p annihilation and medium modifications • Insufficient accuracy, limited power to constrain models Need higher statistics, better resolution Axel Drees

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